Selection of multiple taxonomies in a single field

Some clients use several taxonomies. Many of these can be relevant for a single document. Moving all of the taxonomies into a single taxonomy is not desired as this will make the taxonomy very large and require it to have one workflow for any updates. Documents can have several taxonomy picker fields, but this will push out the document editing form and make it clunky for the editors.

It would be much better for the editors to be able to select from multiple taxonomies from a single field. The field would open the taxonomy picker which would then have a drop down that will be able to switch between multiple taxonomies. The taxonomy terms would then all be saved in a single taxonomy property regardless of which taxonomy they originated from.

The taxonomies that are selectable in the dropdown need to be made configurable, via comma-separated list.